In a current LED drive solution, a conventional high frequency switch mode power supply is mainstream. A high frequency switch circuit is used in the solution, and the circuit is complex and has high costs. Consequently, many vendors begin to study a linear constant current drive solution that is simple, reliable, and cheap.
FIG. 1 shows a linear constant current drive circuit that is currently used, including a mains VAC1, a rectifier bridge DB1, an LED string LED1, and a current source CC1.
The rectifier bridge DB1 has an input end connected to the mains VAC1, and an output end whose positive electrode is connected to an anode of LED1 and whose negative electrode is grounded. A cathode of LED1 is connected to the negative electrode of the rectifier bridge through the current source CC1.
In this solution, a part, of the mains, whose rectified voltage amplitude exceeds a voltage drop of LED1 is all converted into heat to be consumed by the constant current source CC1.
A voltage between two ends of the constant current source CC1 is a residual value obtained after a rectified voltage of the rectifier bridge overcomes the voltage drop of LED1. A larger residual value indicates a larger amount of heat generated by the constant current source CC1, lower efficiency of the circuit. To increase the efficiency, and reduce the amount of generated heat, the voltage drop of the LED needs to be as close as possible to the rectified voltage. However, in this way, when a voltage of the mains decreases, a current passing through LED1 relatively apparently decreases. Therefore, during actual application, a compromise selection needs to be made between the efficiency and an input voltage range. Currently, a relatively common configuration is: Under a 220 VAC input voltage, a total voltage of LED1 is designed to be 230 V to 260 V; under a 120 VAC input voltage, a total voltage of LED1 is designed to be 120 V to 135 V. The configuration applies to different power and different products. This causes a huge difficulty in solution design and cost control of an LED lighting product.
Therefore, a linear constant current circuit structure is needed, to allow a relatively wide range of LED voltages during fixed mains power supplying, so that a lighting designer more flexibly selects an LED.